1. Field
The present invention relates to firearms. More particularly, the present invention relates to automatic, semi-automatic and similar types of rifles and modifications to the rifles.
2. Related Art
There are several problems prevalent in automatic and semi-automatic rifles, such as the family of M-16/AR-15 rifles. The family of M-16/AR-15 rifles discussed herein includes but is not limited to the AR-10, AR-15, M16, M16A1, M16A2, M16A3, M4, M4A1, CAR-15, etc.
FIGS. 1 and 2 illustrate conventional M-16/AR-15 firearms in further detail. As shown in FIGS. 1 and 2, these firearms have an upper receiver 100 with a barrel 4, a front sight 55 on the barrel 4, a handguard 66, and a rear sight 76 on top of the receiver 100. The upper receiver 100 includes a cartridge magazine 103 filled with cartridges 102. In FIG. 1, one cartridge 102 is loaded into the chamber 5a next to the bolt 8 and bolt carrier 10. The firearm also includes a lower receiver 67, which is shown with a trigger 95, trigger guard 96, pistol-style hand grip 72. A shoulder stock 23 is connected to the upper receiver 100 and the lower receiver 67. The firearm also includes a recoil/buffer assembly 17 having a recoil spring 20 mounted in a recoil/buffer tube 21. The recoil/buffer tube 21 extends from and attaches to the lower receiver 67 and is positioned in-line with the barrel 4.
As is shown in FIGS. 1 and 2, the placement of the recoil/buffer assembly 17 directly in-line with the barrel 4 dictates the placement of the shoulder stock 23 in less than ideal positions for the operator. Shoulder stocks 23 for the standard M-16/AR-15 firearms use the recoil/buffer assembly 17 as a structural member and most such structures enclose the recoil/buffer assembly 17. Even if the stock 23 is placed elsewhere, the recoil/buffer assembly 17 cannot move, and sticks out nearly one foot from the back of the receiver 100, which can be awkward for the shooter.
These firearms are operated by a direct gas impingement system, as shown in FIGS. 3-8. The direct gas impingement system directs gas from a fired cartridge to a bolt carrier to cycle the firearm. One major problem with the prior art direct gas impingement system is the venting of hot propellant gases into the receiver areas (i.e., upper receiver 100 and lower receiver 67) of the firearm during operation. In particular, in a standard M-16/AR-15 firearm, hot propellant gas is vented into the upper receiver as the bolt carrier assembly is driven aft and separates from the gas transfer tube. This venting of the propellant gases becomes a problem because the propellant gases carry grimy powder residues and therefore dictate the need for scrupulous and frequent cleaning of virtually all parts of the rifle. Even with frequent cleaning, jamming can occur during long periods of usage. The tube used to deliver these gases into the receiver area also becomes fouled. This small gauge tube, which is difficult to access and clean, can become constricted over time and the resulting lower gas pressure may be insufficient to operate the firearm.
These propellant gases that are vented into the receiver area of the rifle are also very hot. The hot gases enter the receiver area just micro-seconds after being created by an explosion in the cartridge chamber. These hot gases hasten the breakdown of the firearms lubricants and coatings which increases wear, thereby shortening the life of components and increasing the likelihood of jamming.
FIG. 3 illustrates the prior art gas operating system of the M-16/AR-15 firearm in battery just after firing. The gas operating system includes a barrel 4, a bolt carrier assembly 10, a gas block 54, a gas tube 60 and a carrier key 15. In FIG. 3, the bullet 104 is shown traveling down the barrel 4 and is illustrated in a position just before the gas block 54.
FIG. 4 illustrates the firearm's condition just after the bullet has passed the gas block 54. As is seen, the hot, high pressure propellant gas, described above, is routed up through the gas block 54, gas tube 60, and bolt carrier key 15, and into the center of the bolt carrier 10, driving the bolt carrier 15 aft into its recoil position. FIG. 4 also illustrates the venting of contaminating propellant gas 59a into the upper receiver 100 after the carrier key 15 has disengaged from the gas transfer tube 60. This hot, high pressure propellant gas 59a contaminates the inside of the upper receiver 100, coating it with carbon residue and breaking down lubricants. This in turn may cause jamming and shorten the life of components, as described above.
FIGS. 5-8 illustrate the operation of the prior art gas impingement system in further detail. As shown in FIG. 5, the prior art gas impingement system includes a bolt carrier assembly, which includes a bolt carrier 10, bolt carrier key 15, bolt 8, and firing pin 45. The bolt carrier assembly also includes a cam pin 9 to rotate the bolt 8.
As shown in FIG. 6, the burst of expanding high pressure propellant gas 59 from an ignited cartridge traveling up the barrel 4, is routed aft through the gas transfer tube 60, and into a void 11 within the center of the bolt carrier assembly just behind the bolt 8.
As shown in FIG. 7, the pressure of the gas 59 in the void 11 forces the bolt 8 and the bolt carrier 10 in opposite directions, similar to the movement of a piston (i.e, bolt 8) within a cylinder (i.e., bolt carrier 10). The bolt 8 is restrained from moving forward while the bolt carrier 10 moves aft because bolt locking lugs 8a are locked into the barrel extension lugs. The carrier 10 moves aft, directly in line with the barrel and starts to separate the carrier key 15 from the gas transfer tube 60. Then, the carrier 10 engages the bolt cam pin 9 in the bolt cam slot 9a which rotates the bolt to unlock the bolt from the barrel extension. As shown in FIG. 7, the bolt is in an extended, unlocked position.
With reference to FIG. 8, the bolt 8 and bolt carrier 10 are then driven aft together to a full recoil position, helped by the remaining high-pressure gas in the barrel 4. The final travel of the carrier 10 separates the carrier key 15 from the gas transfer tube 60 and vents hot, contaminating, propellant gasses 59a into the upper receiver 100. These vented hot gases coat the inside of the receiver with carbon fouling which, without proper maintenance, can build up and eventually cause jamming and extensive component wear, as described above.
The standard gas system of M-16/AR-15 firearms was originally designed for a rifle having an approximate barrel length of 20″ and having a gas port in the barrel at about 13″ from the receiver. Over the years, the AR-15/M-16 family's barrels have gotten shorter as manufacturers have sought to configure the AR-15/M16 to fit different end user needs. Unfortunately, shortening the barrel and changing the port location changes the operation of the gas system. The placement and size of the gas port and the length of the barrel between the gas port and the forward end of the barrel are an integral part of the operating system design. The distance of the port from the firing chamber, the diameter of the barrel interior, and the power of the cartridge largely determine the gas pressure entering the port as the bullet passes; the size of the gas port determines the gas pressure down stream from the port; the distance of the port from the firing chamber and the distance of the gas path back to the center of the bolt carrier determines the initial gas timing; and, the distance from the gas port to the end of the barrel determines the duration of the gas system pressure.
The timing of the gas system is important, because as the cartridge is fired, the casing's cylindrical walls expand to seal the chamber so the high pressure gases do not vent around the sides of the spent cartridge into the receiver. The spent cartridge stays expanded and stuck in the chamber until the bullet has traveled far enough down the barrel and the pressure drops enough for the casing to contract. The residual gas in the barrel assists in the extraction of the cartridge and supplies some of the energy to move the carrier rearward.
The minimum distance for dependable operation is with the port about 7.5″ from the receiver. Even with that minimum distance, the M-16/AR-15 family of firearms may not function reliably with a full range of ammunition. Some AR-15 style weapons are made with much shorter barrels with gas ports about 4.75″ from the receiver. The gas pressure when the bullet passes the port with the shorter barrels can be as high as 50,000 psi.
This extreme pressure traveling in such a short gas path initiates the carrier's action before the empty casing has had time to contract away from the walls of the chamber. The firearm may function most of the time, but the high pressures often causes problems. For example, the bolt's case extractor is exposed to increased stress because the extractor tries to pull the stuck case out by the case rim, subjecting the extractor to breakage. In another example, the extractor sometimes rips the back off of the spent case. In addition, if the extractor spring is not strong enough, the extractor can slip off of the cartridge rim. Also, if the spring is too strong, the extractor may not slip into place over the rim when the cartridge is loaded into the chamber.
Another problem with the prior art M-16/AR-15 rifles is that the shoulder stock does not sit comfortably or properly against the shooter's shoulder, which does not allow for efficient absorption of recoil energy or for comfortable rifle handling. In an upright shooting stance, up to half of the upper part of the stock end is above and not in contact with the shooters shoulder. The most efficient transfer of recoil energy is to spread it over as large an area as possible. The felt recoil from the 0.223/5.56 mm cartridge is not great, but with the M-16/AR-15 now being adapted for much more powerful ammunition, the handling of recoil energy is becoming more important to the shooter.
FIG. 9 illustrates a man preparing to fire a prior art firearm in the M-16/AR-15 family. In particular, FIG. 9 shows how the original M-16/AR-15 style stock 23 sits high on the shooters shoulder 80 in a common shooting stance. As described above, the stock 23 cannot be moved lower on the firearm because the recoil/buffer tube 21 extends into the shoulder stock 23.
FIG. 10 shows a prior art M-16/AR-15 style firearm illustrating that the placement of the recoil/buffer tube 21 at the top of the shoulder stock 23 sets the placement of the stock 23 high on the firearm. In the M-16/AR-15 style of firearms, the top of the shoulder stock 23 is on a slightly higher horizontal plane than the top of the barrel 4. Because of the height of the stock 23, the shooter's head and eye line 77 cannot get close to barrel 4. This raises the normal sightline 77 to more than 2″ above the barrel centerline, which causes inefficient parallax. This parallax is particularly evident when the shooter shifts his point-of-aim from a close target to a distant one, or the reverse. In this case, the projectile's point-of-impact changes dramatically in relation to the point-of-aim unless the sights are adjusted for the change in distance. Parallax is typically not a problem for target shooters who shoot at a single distance; however, parallax can be a significant problem for hunters, action competition shooters, law enforcement and the military. The relationship 79 between the sightline 77 and the stock 23 and the distance 78 between the barrel 4 and the sightline 77 are also illustrated in FIG. 10. As shown in FIG. 10, because of the rear mounted recoil tube, recoil spring and buffer assembly, the standard M-16/AR-15 is a relatively long weapon.
Other firearms, such as the AK-47 and FAL, use piston driven gas operating systems. The piston driven gas operating systems do not vent operation gases into their receivers. Instead, propelling gasses drive a piston which in turn drives a piston rod. This piston rod impacts and drives the bolt carrier assembly of the weapon. Although the gas piston operating system leaves the receiver cleaner and cooler, the gas piston operating system induces vibration and flexes the barrel. The power to operate gas piston systems is delivered off-line from the barrel which causes the barrel to flex and vibrate each time a cartridge is fired. This flex and vibration is the reason that firearms having gas piston systems are inherently less accurate than firearms having direct gas impingement systems.